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1988 944 Base with R12. AC compressor not engaging. It took me a while, but I determined that the snowflake switch and G17 relay are working. I confirmed this by measuring 12 volts at the input of the low pressure switch when I pressed the snowflake switch. The compressor will engage if I hot wire it (connect to batt charger). So I am assuming that the system is low on R12, and finally convinced myself to convert to R134a. Was planning on replacing the o-rings and dryer and adding the correct oil to the compressor.
But the weird thing is that the gap between the clutch and the compressor is very small, less than .010". I could only get the .010" feeler gage to slip in the gap in a few places (when not engaged). The attached picture is when the clutch is NOT engaged. It was difficult to tell it was engaged when I applied the 12v, because I couldn't see the clutch move. I could only tell by trying to spin the clutch when I applied the 12v and it was definitely engaged. The resistance of the clutch is 3.1 ohms.
Is this small gap normal? I thought it should be closer to like .030"-.040"? Should I try to fix this when I pull the compressor out to replace o-rings? Although, I'm not sure exactly how I would go about fixing it
The spec clearance there considers how much the friction material might 'grow' when hot or wet, vs. how hard it might be to have the coil pull the metal drive plate back against that friction material surface. Your 0.010" clearance is really small, and when they drag you'd know by the burn marks, the swelling, and the fragrance while driving. Your burned-off paint is a serious telltale of heat from dragging.
The coil resistance on my same-period same-manufacturer likely same-model compressor is about 6 Ohms, more than a measurement-error higher than you report. It's quite possible that the clutch coil is damaged internally by all that heat.
FWIW, in my 'do everything together all at once' philosophy on maintenance like this... If you do decide to just add a shim there, I'd replace the clutch bearing at the same time. From the evidence you've presented, I'd be shopping for at least a complete clutch, and more likely a whole compressor. The compressor itself is a somewhat generic Denso piece, and you swap the hose manifold from your old one to make it the correct Porsche unit. Shop that way, and it will be a LOT less Porsche-tax $$ than searching for a 944-specific compressor.
If you just replace the clutch, there's a very good argument for replacing the case o-rings and the front shaft seal for refrigerant oil compatibility too. You'll replace the o-rings under the manifold plate regardless. There are plenty of good YouTube guides for these Denso compressors, and the parts are very readily available, and not at all expensive. Otherwise, a new or well-rebuilt compressor has all the right stuff in it for R-134a oil. If you are going to flush the rest of the system to clear all the oil out (a Good Idea...), you can use the PAG oil that will be in the new compressor. If no flush, then swap that for POE, which doesn't turn the remaining original mineral oil to jelly. The oils are both hygroscopic, so best to install the compressor and the dryer last, immediately before you plan to pull vacuum. Keep these two parts sealed up until installation.
Dr Bob, Good insight on the clutch and/or compressor likely being damaged due to the clutch dragging and the heat. I don't think im up for rebuilding it. You mentioned a generic Denso could work, connecting to my manifold plate. Is there a compatible non-Porsche p/n I can search for?
I was also considering getting a used compressor from a reputable 944 part supplier. Thoughts?
My similar-period car uses IIRC a PA15 Denso, only specific is the hoses at the manifold plate. It's used on many cars all the way to farm implements, just with different hose connections. The community here can offer some 944-specific info on exactly which one came on your car. For a while I was on the path to grabbing a new generic as a shelf spare for mine, but so long as mine is working and the parts are still so easily available, no spare here yet. So far so good, and still super cold a local club tour this week.
The gap is too tight should be around 0.020"–0.040. A 0.010" gap (or less) can cause drag and premature wear. Since you're already pulling the compressor for the R134a conversion, now’s the time to adjust it. You’ll need to remove the clutch and either shim it properly or file down the spacer. Check the clutch plate and pulley for wear while you're in there. Resistance (3.1Ω) is fine, so focus on the gap.
I took off the clutch and put a washer in to get the gap closer to the nominal. The odd thing is no oil came out of the compressor. Does that mean the compressor is damaged?
